The invention relates to a drill composed of hard metal having a diameter of a maximum of 1.5 mm for drilling contact holes in multilayer synthetic circuit boards. The drill has helical flutes in its twisted portion and two main and subsidiary cutting edges which are arranged opposite the drill axis, and at least one of these cutting edges possesses a different angle or a different length relative to the other similar cutting edge as described in German Patent Application No. P2655452.9, incorporated herein by reference. The difference between the two cutting angles or cutting lengths to be compared with one another is selected to be such that the opening of the driled hole at the drill inlet surface of the circuit boards amounts to at least 20 .mu.m, but not more than 50 .mu.m.
With materials which, when drilled, do not form continuous chips but drilling dust, the danger exists that the drilling dust wil be pressed against the walls of the bore by the subsidiary cutting edges of the drill where it will form a fixed film. This contamination is extremely difficult to remove. In the drilling of multi-layer circuit boards, the "smearing" of the contact holes leads to impure contacts between the individual inner layers. Therefore it has already been proposed in German Offenlegungsschrift No. 1,704,296 (incorporated herein by reference) to rapidly discharge the drilling dust by using a jet of compressed air by means of which the drilling dust is blown out of the contact holes during drilling or following drilling. High drilling speeds are used to drill contact holes of this type which have a diameter of a maximum of 1.5 mm in circuit boards composed of synthetic material; therefore twist drills composed of sintered material, so-called "hard metal" are used to drill the contact holes. In order to reduce smearing during drilling, so-called head drillers are used, whose twisted portions, which bear the flutes, behind the headpart are set back by approximately 0.006 mm. The length of the head part is about 0.6mm. This technique is utilized to prevent drilling dust being pressed against the walls of the bore, Nevertheless, this structural technique serves only to reduce, not, however, to eliminate the smearing.
The bit of the drill is formed by the transverse cutting edge which is adjoined by the main cutting edges on both sides of the central axis of the bore. These main cutting edges lead to the cutting edges of the twist drill which are adjoined by the helical subsidiary cutting edges which define the flutes. The transverse cutting edge is subject to the greatest load as it cuts less into the material but exerts a frictional action upon the material to be drilled.
During the regrinding of twist drills of all types, it can occur that the center of the main cutting edges or the center of the transverse cutting edges lies outside of the drill axis or that the main cutting edges possess unequal lengths or an unequal angle to one another. This has the disadvantage as noted in Huette Handbook for Managing Engineers, edited by Wilhelm Trust & John Volume I, 6th edition, 1964 page 537d Berlin, Munich, that the cutting edges are subject to unequal loads and there is an increase in the drilling dust during drilling. Furthermore the danger exists that the dril will slip. Consequently, an asymmetrical drill grinding surface is a disadvantage which should be avoided as far as possible. Therefore the dimensional differences between the grinding angles necessary during the grinding of the drills should not exceed 2.degree. in the case of small drills of the type described in the introduction.
It has already been proposed that the lengths or angles of the cutting edges located on each side of the axis of the twisted portion of the drill should differ from one another. However, it is difficult to produce drill grinding surfaces of this type which are easily reproducible. Furthermore it is time consuming to measure drill grinding surfaces of this type in order to check the product.